Alkaline AA batteries are rated 1.5 volts, while NiMH AA batteries are rated at 1.2 volts. NiMH generally refers to a battery being rechargeable and to the materials from which the battery is constructed.
The difference of voltage ratings between the two types of battery is because the alkaline battery starts at 1.5 volts but depletes over time, giving it an average voltage of 1.2 volts, which is the same as the NiMH battery. On the other hand, NiMH batteries deliver a consistent 1.2 volts throughout their life-cycle. It is important to note that while these batteries are rated at 1.2 volts or 1.5 volts, the amount of voltage they actually emit may be lower, depending on battery brand and quality of construction.